Japanese quail inoculated with mlMSV (BMUX) infected quail cells developed fibrosarcomas at the site of injection. In a cell line derived from a quail tumor the expression of the mlMSV specific pP60gag polyprotein previously detected in all infected mammalian cells tested was modified. In this cell line, the MSV specific polyprotein was smaller, pP58gag, and rapidly processed. The observed changes may provide insight into the question of how virus transformation-associated polyproteins are metabolized.